"AKC SCIENCE CLASSES"
CLASS 10 TH (CBSE AND MP BOARD)
CHAPTER 12
ELECTRICITY
INTEXT QUESTIONS AND ANSWERS
NCERT INTEXT QUESTIONS (PAGE NO. 200)
Q.01:- What does an electric circuit mean?
Ans:- A continuous and closed path along which an electric current flows is called an electric circuit.
Q.02:- Define the unit of current.
Ans:- If one coulomb of charge flows through any section of a conductor in one second, then current through it is said to be one ampere.
1 A = 1 Cs⁻¹
Q.03:- Calculate the number of electrons constituting one coulomb of charge.
Ans:- Charge on one electron,
e = 1.6 ✖ 10⁻¹⁹ C
Number of electrons, n = Q/e
n = 1C/(1.6 ✖ 10⁻¹⁹ C) = 6.25 ✖ 10¹⁸. Ans.
NCERT INTEXT QUESTIONS (PAGE NO. 202)
Q.01:- Name a device that helps to maintain a potential difference across a conductor.
Ans:- A battery.
Q.02:- What is meant by saying that the potential difference between two points is 1V?
Ans:- The potential difference between two points is 1 volt if one joule of work is done in moving a positive charge of one coulomb from one point to the other point.
Q.03:- How much energy is given to each coulomb of charge passing through a 6V battery?
Ans:- Energy given by battery
= Charge ✖ Potential difference
= 1 C ✖ 6 V = 6 J.
NCERT INTEXT QUESTIONS (PAGE NO. 209)
Q.01:- On what factors does the resistance of a conductor depend?
Ans:- The resistance of a conductor depends :-
- on its length,
- on its area of cross-section,
- its temperature and
- on the nature of its material.
Q.02:- Will current flow more easily through a thick wire than a thin wire of the same material, when connected to the same source? Why? Ans:- The current will flow more easily through a thick wire than a thin wire of the same material. Larger the area of cross-section of a conductor of a conductor, more is the ease with which the electrons can move through the conductor. Hence smaller is the resistance of the conductor.
Q.03:- Let the resistance of an electrical components remains constant while the potential difference across the two ends of the component decreases to half of its former value. What change will occur in the current through it?
Ans:- When potential difference is halved, the current through the component also decreases to half of its initial value. This is in accordance with Ohm's law
i.e., V ∝ I.
Q.04:- Why are coils of electric toasters and electric irons made of an alloy rather than a pure metal?
Ans:- The coils of electric toasters and electric irons are made of alloys instead of pure metal due to the following reasons :-
- Alloys have higher resistivity than that of their constituent metals.
- Alloys do not oxidise (or burn) readily at high temperatures.
Q.05:- Use the data in Table 12.2 (NCERT TB) to answer the following :-
(a) Which among the iron and mercury is better conductor ?
(b) Which material is the best conductor ?
Ans:- (a) Resistivity of iron = 10.0 ✖ 10⁻⁸ Ω m
Resistivity of mercury = 94.0 ✖ 10⁻⁸ Ω m.
Thus iron is a better conductor because it has lower resistivity than mercury.
(b) As silver has the lowest resistivity (= 1.60 ✖ 10⁻⁸ Ω m), so silver is the best conductor.
NCERT INTEXT QUESTIONS (PAGE NO. 213)
Q.01:- Draw a schematic diagram of a circuit consisting of a battery of three cells of 2V each, a 5 Ω resistor, and a 12 Ω resistor, and a plug key, all connected in series.
Ans:- The required circuit diagram is given below :-
Q.02:- Redraw the circuit of Question 1, putting in an ammeter to measure the current through the resistors and a voltmeter to measure the potential difference across the 12 Ω resistor. What would be the readings in the ammeter and the voltmeter?Ans:- The required circuit diagram is given below :-
Total voltage, V = 3 ✖ 2 = 6 VTotal resistance, R = 5 + 8 + 12 = 25 Ω
Reading of ammeter, I = V/R = 6/25 = 0.24 A. Ans.
Reading of voltmeter, V = I ✖ R = 0.24 ✖ 12 = 2.88 V. Ans.
NCERT INTEXT QUESTIONS (PAGE NO. 216)
Q.01:- Judge the equivalent resistance when the following are connected in parallel :
(a) 1 Ω and 10⁶ Ω,
(b) 1 Ω, 10³ Ω and 10⁶ Ω.
Ans:- When the resistances are connected in parallel, the equivalent reistance is smaller than the smallest individual resistance.
1 ✖ 10⁶
(a) Equivalent resistance, R = -------------- < 1 Ω
1 + 10⁶
(a) Equivalent resistance,
1 ✖ 10³ ✖ 10⁶
R = --------------------------------------------- < 1 Ω
1 + 10⁶ + 1 ✖ 10⁶ + 1 ✖ 10³
Q.02:- An electric lamp of 100 Ω, a toaster of resistance 50 Ω, and a water filter of resistance 500 Ω are connected in parallel to a 220 V source. What is the resistance of an electric iron connected to the same source that takes as much current as all three appliances, and what is the current through it?
Ans:- Resistance of electric lamp, R1 = 100 Ω
Resistance of toaster, R2 = 50 Ω
Resistance of water filter, R3 = 500 Ω
Equivalent resistance Rp of the three appliances connected in parallel is given by
1 1 1 1
-------- = ------- + -------- + -------
Rp R1 R2 R3
1 1 1 16
= --------- + -------- + -------- = ----------
100 50 500 500
Or
500
Rp = --------- Ω = 31.25 Ω .
16
Resistance of electric iron = Equivalent resistance of the three appliances connected in parallel
= 31.25 Ω .
Applied voltage, V = 220 V
Current, I = V/R = 220V/31.25 Ω = 7.04 A. Ans.
Q.03:- What are the advantages of connecting electrical devices in parallel with the battery instead of connecting them in series?
Ans:- The advantages of connecting electrical devices in parallel with the battery are as follows :-
- Each device gets the full battery voltage.
- The parallel circuit divides the current through the electrical devices. Each device gets proper current depending on its resistance.
- If one device is switched OFF/ON, other are not affected.
Q.04:- How can three resistors of resistances 2 Ω, 3 Ω, and 6 Ω be connected to give a total resistance of (a) 4 Ω,
(b) 1 Ω?
Ans:- (a) We can obtain a total resistance of 4 Ω by connecting the 2 Ω resistance in series with the parallel combination of 3 Ω and 6 Ω.
R2R3 3 ✖ 6
R = R1 + ----------------- = 2 + ---------------- = 4 Ω. Ans.
R2 + R3 3 + 6
(b) We can obtain a total resistance of 1 Ω by connecting resistances of 2 Ω, 3Ω and 6 Ω in parallel.
1 1 1 1
------- = ------- + ------- + -------
R R1 R2 R3
1 1 1 1 1
-------- = -------- + -------- + -------- = -------
R 2 3 6 1
Or R = 1 Ω. Ans.
Please note, this type of questions can be solved by trial and even only.
Q.05:- What is (a) the highest, (b) the lowest total resistance that can be secured by combinations of four coils of resistance 4 Ω, 8 Ω, 12 Ω, 24 Ω ?
Ans:- (a) Highest resistance can be obtained by connecting the four coils in series. Then
R = 4 + 8 + 12 + 24 = 48 Ω. Ans.
(b) Lowest resistance can be obtained by connecting the four coils in parallel. Then
1 1 1 1 1
-------- = -------- + -------- + -------- + --------
R 4 8 12 24
12 1
= -------- = --------
24 2
Or R = 2 Ω.
NCERT INTEXT QUESTIONS (PAGE NO. 218)
Q.01:- Why does the cord of an electric heater not glow while the heating element does?
Ans:- Both the cord and the heating element of an electric heater carry the same current. But the heating element becomes hot due to its high resistance (H= I²Rt) and begins to glow. The cord remains cold due to its low resistance and does not glow.
Q.02:- Compute the heat generated while transferring 96,000 coulomb of charge in one hour through a potential difference difference of 50 V.
Ans:- Here Q = 96,000 C, t = 1 hour = 3600 s,
V = 50 V.
Heat generated is (H) = VQ = 50V ✖ 96,000 C
= 48,00,000 J. Ans.
Q.03:- An electric iron of resistance 20 Ω takes a current of 5 A. Calculate the heat developed in 30s.
Ans:- Here, R = 20 Ω, I = 5 A, t = 30s
Heat developed is
H = I²Rt = 25 ✖ 20 ✖ 30 = 15,000 J. Ans.
NCERT INTEXT QUESTIONS (PAGE NO. 220)
Q.01:- What determines the rate at which energy is delivered by a current?
Ans:- Resistance of the circuit determines the rate at which energy is developed by a current.
Q.02:- An electric motor takes 5 A from a 220 V line. Determine the power of the motor and the energy consumed in 2 h.
Ans:- Here, I = 5A, V = 220V, t = 2 h = 7200s
Power, P = VI = 220 ✖ 5 = 1100 W. Ans.
Energy consumed = P ✖ t = 1100 W = 7200 s
= 7920000 J. Ans.
